Spring migration is coming to an end. Bringing up the rear are Black-billed Cuckoos, Yellow-bellied Flycatchers, Nelson’s Sparrows and Saltmarsh Sparrows. The north-flowing river of birds is running dry.
We see an almost synchronous arrival of a guild of songbirds collectively called the leaf-gleaning insectivores. These birds include our vireos, warblers and tanagers. All of them make a living by preying on caterpillars and other herbivores that attack the leaves of deciduous trees. The leaf-gleaning herbivores are the friends of the trees, gobbling up the leaf-eating insects. A cascade of events occurs in spring allowing the warblers and vireos to return: leaf-out, followed by emergence of caterpillars, followed by the arrival of the leaf-gleaning birds. In central and southern Maine, the first ten days of May capture the arrival of many of these birds.
Among these arrivals are Red-eyed Vireos. I daresay that Red-eyed Vireos vie for the title of most common woodland bird in eastern North America. A bird of treetops, Red-eyed Vireos are much more often heard than seen.
Hearing a Red-eyed Vireo is a snap because they sing vigorously all through the day. Their song is a series of two- and three-note phrases. An effective mnemonic for learning the song is “here-I-am, where-are-you, over-here, in-the-tree”.
The song is rather monotonous and dry. Despite the seeming monotony of their song, Red-eyed Vireos show remarkable diversity in their two- and three-note phrases. A typical Red-eyed sings around 45 phrases. Those phrases are strung together to make a distinctive song type. Each song type consists of the same one to five phrases. A typical male sings about 30 song types.
A less common vireo breeding in Maine, the Philadelphia Vireo, needs to be considered in this column. Red-eyed and Philadelphia vireos share an intriguing overlap in their song.
The Philadelphia Vireo closely resembles the Red-eyed Vireo but has a less distinct line above the eye and has a yellow wash on the underparts. The Philadelphia Vireo is also smaller, averaging 12 grams in weight to the 17-gram weight of a typical Red-eyed Vireo.
Most nesting male songbirds defend their territories against other males of its species but not against males of other species. However, Red-eyed Vireos and Philadelphia Vireos defend their territories against their own species and against the other species.
The song of the Philadelphia Vireo is very similar to the song of the Red-eyed Vireo song. Even highly experienced birders pass off singing Philadelphia Vireos as the more common Red-eyed Vireos. The reason for the similarity will soon be apparent.
In northern New England forests, insect prey may become quite hard to find during the breeding season. Because both vireos eat the same insects, there is an advantage for a territorial vireo to keep a member of its own species and members of the other vireo species away from its food sources.
In most cases, the vireos avoid direct confrontations over the boundaries of a territory. Instead, a territorial bird proclaims his ownership of a territory by singing from perches throughout his territory. Similarly adjacent territory owners sing throughout their territory. The neighboring birds recognize unseen but real boundaries, avoiding physical interactions.
The problem the Philadelphia Vireo has is how to maintain exclusive ownership of a territory, defending against a larger and stronger Red-eyed Vireo that may be trying to expand his territory. Philadelphia Vireos have solved the problem by becoming a social mimic. These birds mimic the song of the Red-eyed Vireo.
Play-back experiments have shown that Red-eyed Vireos cannot tell the difference between a Red-eyed Vireo song and a Philadelphia song. No wonder birders have trouble telling the two species apart by song! On the other hand, Philadelphia Vireos can distinguish between a Philadelphia Vireo song and a Red-eyed Vireo song.
Philadelphia Vireos mimic the song of the Red-eyed Vireo to level the playing field; it’s a case of deception over brawn.